Acoustical investigation can be used to investigate casings and mediums used in completions of boreholes. Oilfield acoustical investigation may involve, for example, the use of ultrasonic waves.
In the context of wellbore completions, boreholes penetrating an earth formation may include a formation wall with a casing disposed therein. An annular space (or annulus) positioned between the formation wall and the casing may be filled with a medium made of various materials, such as cement, drilling fluids, muds and/or other annulus solids. After the medium has set in the annular space, acoustic testing methods may be used to evaluate the integrity of the medium. The medium may impose zonal isolation between different formations in order to avoid flow of fluids (e.g., water, gas, oil) from the formations through the annular space of the casing. Medium placement and integrity may also affect a well's architecture, for example, to ensure mechanical support of the casing and/or to provide protection from corrosion.
Cement evaluation may be used to determine, for example, a location and/or presence of various materials that form the medium in the annular space. Various tools, such as those using acoustic waves, may be used to probe the medium outside the casing. These tools may probe based on the shear impedance of the medium (roughly the product of the density of the medium and the shear velocity of the acoustic waves), compressional impedance of the medium (roughly the product of the density of the medium and the longitudinal velocity of the acoustic waves), and/or combinations thereof. Such evaluation may be used to detect potential problems that may arise, for example, when a bond between the medium and the casing has failed, when the medium has not fully set, when light weight and/or low density cements are used in the medium, and other practical scenarios in which conditions are less than ideal, thus potentially leaving a range of annulus solids in the medium.